According to Chang what happened to the Russian man who still looked young though he was almost eighty when forced to leave Shangri-La? (chapter nine)

The Russian had barely aged at all during his time in Shangri-La. When he left to help search for a party of travelers, he was taken prisoner by a nomadic tribe. Over the three months he was held, the man aged and soon died.

This man, a Russian, had arrived here originally in the prime of life, and had taken to our ways so well that at nearly eighty he did not look more than half as old. He should have been absent no longer than a week (which would not have mattered), but unfortunately he was taken prisoner by nomad tribes and carried away some distance. We suspected an accident and gave him up for lost. Three months later, however, he returned to us, having made his escape. But he was a very different man. Every year of his age was in his face and behavior, and he died shortly afterwards, as an old man dies.”